1. Field of the Invention
This invention relates to cooling systems and, more specifically, to cryogenic cooling systems.
2. Description of the Related Art
Propulsion systems utilizing cryogenic liquid oxygen and/or hydrogen, such as the Space Shuttle, Atlas/Centaur, Delta, etc., are currently filled from the facility storage tanks and subsequently allowed to cool in the flight tanks in order to reject the heat absorbed by the liquid as a result of environmental heat leak, transfer line, and tank wall chill-down. The cooling of the liquid bulk is desirable in order to increase the liquid density so that more impulse mass can be stored in the tank, and also to reduce the liquid vapor pressure so that the tank operating pressure and tank weight is minimized.
The prior art discloses numerous process and systems for cooling the cryogenic liquid. The cryogenic liquid may be cooled through jackets on the tanks. However, due to weight restrictions and the problems associated with such low temperature cooling, a cryogenic liquid stream is cooled. The cryogenic liquid stream that initially charges the tank may be cooled prior to entering the tank. The cryogenic liquid in the tank is furthered cooled through reducing the temperature of a recirculation stream of cryogenic fluid.
The prior art discloses a cryogenic heat exchanger system that comprises a tube or tubes that extend through a reservoir. The cryogenic stream that is to be cooled is directed through the tubes. The reservoir is filled with another cryogenic fluid at a lower temperature. The tubes through which the cryogenic fluid stream flows are submerged in the reservoir, resulting in the cryogenic fluid stream being cooled as it moves through the tubes.
The cryogenic fluid in the heat exchanger reservoir may be at a sub-atmospheric pressure to maintain it at a lower temperature and assist in thermal transfer between it and the cryogenic fluid stream. The prior art discloses using a dedicated compressor to create the sub-atmospheric pressure and compress the vapor from the reservoir prior to discharge. This process and system directly requires the use of the compressor, which is a complex piece of equipment.
Therefore, a need exists to create a sub-atmospheric pressure in the heat exchanger reservoir system that requires simpler dedicated equipment than a compressor.